A method for removing nitrogen oxides from a gas using an iron exchanged zeolite catalyst

ABSTRACT

A method for removing nitrogen oxides NOx from a gaseous current, comprising the steps of: passing the gaseous current through a de-NOx catalytic bed with iron exchanged zeolite as a catalyst with the addition of ammonia as a reducing agent, wherein the molar ratio of NH3 over NOx is greater than 1.33.

FIELD OF THE INVENTION

The invention relates to systems for removing nitrogen oxides from agas.

PRIOR ART

Removal of nitrogen oxides (NOx) from a gas is of interest forenvironmental reasons. The flue gas of a combustion process typicallycontains NOx due to nitrogen and oxygen in the oxidant air reacting atthe high temperature of combustion. A further case of interest is theremoval of NOx from the tail gas of nitric acid production. Typically,in the nitric acid production, the tail gas denotes the gaseous currentwithdrawn from an absorption column where NOx is absorbed into water toproduce nitric acid. The tail gas contains residual amounts of NOx whichmust be removed before the gas can be discharged.

A known measure to remove NOx from a gaseous current is passing theNOx-containing gas through a suitable catalytic bed with ammonia as areducing agent. In the presence of ammonia, the nitrogen oxides arecatalytically reduced to N2 and H2O.

The addition of ammonia needs be carefully regulated to reach a targetremoval of NOx while avoiding ammonia slip, i.e. that some ammoniaescapes the catalytic bed. A content of ammonia in the output gasleaving the catalytic bed is undesirable, especially if the effluent gasof the catalytic bed is then discharged to atmosphere. The permissiblecontent of ammonia in a gas discharged into atmosphere is generally verylow, for example 5 ppm. In some countries, release of ammonia intoatmosphere may be subject to a penalty fee.

Ammonia slip may also lead to formation of undesirable compounds such asammonium nitrite or nitrate which can damage downstream equipment. Forexample the tail gas of nitric acid production is under pressure and isgenerally sent to an expander after removal of NOx. The above compoundsmay damage the expander.

SUMMARY OF THE INVENTION

It has been surprisingly found that ammonia slip is unexpectedly low, ornegligible, when a NOx-containing gaseous current is passed through ade-NOx catalytic bed, wherein the catalytic bed comprises iron-exchangedzeolite catalyst and the molar ratio of NH3 over NOx in the gas admittedto the catalytic bed is relatively high, being greater than 1.33.

Accordingly, an object of the present invention is a method for removingNOx from a gaseous current according to claim 1. Preferred embodimentsare disclosed in the dependent claims.

The term NOx denotes collectively NO and NO2.

The iron-exchanged zeolite catalyst, also termed iron-laden zeolitecatalyst, is preferably any of: MFI, BEA, FER, MOR, FAU, MEL, or acombination thereof. Preferably, said iron exchanged zeolite is of theFe-ZSM-5 type.

Preferably said ratio of NH3 over NOx is greater than 1.4, morepreferably being 1.4 to 2, more preferably 1.4 to 1.6, even morepreferably about 1.5. Preferred values include any of: 1.4, 1.45, 1.5,1.55, 1.6.

After the passage though said de-NOx catalytic bed, the residual amountof NOx in the gas is preferably not greater than 100 ppm, preferably notgreater than 50 ppm, more preferably not greater than 25 ppm. The termppm denotes parts per million in volume.

In a preferred embodiment, said de-NOx catalytic bed is operated at atemperature in the range of 400 to 450° C., preferably 430° C. or around430° C. Preferred working temperatures of the catalytic bed include:420° C., 425° C., 430° C., 435° C.

The space velocity in said de-NOx catalytic bed is preferably 10000 h⁻¹to 14000 h⁻¹, preferably 10000 to 13000 h⁻¹ and more preferably 13000h⁻¹.

In some embodiments the NOx-containing gas is under pressure. In someembodiments the absolute pressure of the gas in said de-NOx catalyticbed is greater than 1 bar, preferably greater than 2 bar, morepreferably 2 to 25 bar, even more preferably 5 to 15 bar.

The NOx-containing input gaseous current can be a flue gas of acombustion process, or a tail gas of a process for making nitric acid,namely a gas withdrawn from an absorption column. Removing environmentalhazardous compounds from such tail gas (downstream of absorption column)is also termed tertiary abatement in contrast with primary and secondaryabatement which are performed upstream the absorption column, orquaternary abatement which is performed after a subsequent expansion ofthe tail gas through an expander.

In an embodiment, the method of the invention does not comprise passingthe NOx-containing gas (e.g. combustion flue gas or tail gas of nitricacid production) through a de-N2O catalytic bed before the passagethrough said de-NOx catalytic bed.

In an embodiment, the method of the invention does not comprise passingthe NOx-containing gas (e.g. combustion flue gas or tail gas of nitricacid production) through a series of a further de-NOx catalytic bed anda subsequent de-N2O catalytic bed before the passage through said de-NOxcatalytic bed.

The NOx-containing gas, however, may be passed through a first de-NOxcatalytic bed and then into said de-NOx catalytic bed. An embodiment ofthe method of the invention includes: passing the NOx-containing gasthrough a first de-NOx catalytic bed, adding ammonia as a reducing agentto the effluent of said first-de-NOx catalytic bed until the molar ratioof NH3 over NOx in said effluent gas is greater than 1.33, preferably1.4 to 1.6, passing the effluent gas and ammonia directly through saidde-NOx catalytic bed without a passage through a de-N2O catalytic bed.

In some embodiments the ammonia can be pure or in the form of a reducingagent containing ammonia.

The catalytic bed of the invention can be contained in a suitable vesseland can be traversed with axial, radial or mixed axial/radial flow,according to different embodiments. More than one catalytic bed, ifprovided, can be arranged in the same pressure vessel or separatepressure vessels. More than one catalytic bed contained in a singlepressure vessel can be arranged one above the other or concentrically.

The invention provides a certain excess ammonia in the de-NOx catalyticbed, the amount of ammonia being more than 1.33 moles per mole of NOx.

The applicant has found that, in a surprising manner, the combination ofiron-laden zeolite catalyst and of the above NH3/NOx ratio, particularlywith an operating temperature of the catalyst around 430° C., results ina virtual absence of ammonia slip, typically less than 1 ppm. At thesame time, the nitrogen oxides are efficiently removed. Accordingly theinvention provides a method which is able to meet the most stringentlimits of NH3 and NOx for emission into atmosphere.

The invention will be now further elucidated with reference to anon-limitative example.

DETAILED DESCRIPTION

A NOx-containing gas, which can be a combustion flue gas or a tail gasof nitric acid production, is passed through a de-NOx catalytic bed withthe addition of ammonia as a reducing agent and NH3/NOx ratio in the gasgreater than 1.33.

The NOx-containing gas may be admitted directly to said de-NOx catalyticbed or, in some embodiments, can be subject to a preliminary treatmente.g. in another de-NOx catalytic bed.

For example, a gas containing 200 ppm of NOx and 10 ppm of N2O, ispassed through a de-NOx iron-laden zeolite catalytic bed with a spacevelocity of 13000 h⁻¹ at a pressure of 7 bar (absolute) and atemperature of 430° C. The gas is a tail gas of nitric acid productionand further contains 3% oxygen and around 0.3% water. The NH3/NOx ratiowas varied between 1.33 and 1.5.

Abatement of 99.4% NOx was observed with a NH3/NOx ratio of 1.4 and99.7% was observed with a NH3/NOx ratio of 1.5. The ammonia content ofthe effluent gas was below measurement level, i.e. no ammonia slip wasdetected.

1-11. (canceled)
 12. A method for removing nitrogen oxides NOx from agaseous current, the method comprising: passing the gaseous currentthrough a de-NOx catalytic bed including a catalyst that is an ironexchanged zeolite, with an addition of ammonia as a reducing agent;wherein a molar ratio of NH3 over NOx in the gas admitted to said de-NOxcatalytic bed is 1.4 to 2; wherein said de-NOx catalytic bed is operatedat a temperature in a range of 420° C. to 435° C.; and wherein a spacevelocity in said de-NOx catalytic bed is 10000 h⁻¹ to 14000 h⁻¹ . 13.The method according to claim 12, wherein said molar ratio of NH3 overNOx is 1.4 to 1.6.
 14. The method according to claim 13, wherein saidmolar ratio of NH3 over NOx is 1.5.
 15. The method according to claim12, wherein, after the passage though said de-NOx catalytic bed, aresidual amount of NOx in the gas is not greater than 100 ppm.
 16. Themethod according to claim 12, wherein, after the passage though saidde-NOx catalytic bed, a residual amount of NOx in the gas is not greaterthan 50 ppm.
 17. The method according to claim 12, wherein, after thepassage though said de-NOx catalytic bed, a residual amount of NOx inthe gas is not greater than 25 ppm.
 18. The method according to claim12, wherein the temperature at which said de-NOx catalytic bed isoperated is 430° C.
 19. The method according to claim 12, wherein theiron exchanged zeolite catalyst includes MFI, BEA, FER, MOR, FAU, MEL,or combinations thereof.
 20. The method according to claim 12, whereinthe iron exchanged zeolite includes an Fe-ZSM-5 type iron exchangedzeolite.
 21. The method according to claim 12, wherein the spacevelocity in said de-NOx catalytic bed is 13000 h⁻¹.
 22. The methodaccording to claim 12, wherein the gas in said de-NOx catalytic bedexhibits an absolute pressure of greater than 1 bar.
 23. The methodaccording to claim 22, wherein the absolute pressure is 2 bar to 25 bar.24. The method according to claim 22, wherein the absolute pressure is 5bar to 15 bar.
 25. The method according to claim 12 wherein the gaseouscurrent is a tail gas of a process for making nitric acid, withdrawnfrom an absorption column.
 26. The method according to claim 12, furthercomprising: passing the NOx-containing gas through a first de-NOxcatalytic bed; adding ammonia as a reducing agent to the effluent ofsaid first-de-NOx catalytic bed until the molar ratio of NH3 over NOx insaid effluent gas is 1.4 to 2; and passing the effluent gas and ammoniadirectly through said de-NOx catalytic bed without a passage through ade-N2O catalytic bed.
 27. The method according to claim 26 whereinadding ammonia as a reducing agent to the effluent of said first-de-NOxcatalytic bed until the molar ratio of NH3 over NOx in said effluent gasis 1.4 to 2 includes adding ammonia as the reducing agent to theeffluent of said first-de-NOx catalytic bed until the molar ratio of NH3over NOx in said effluent gas is 1.4 to 1.6.
 28. The method according toclaim 12, wherein the gaseous current is a flue gas of a combustionprocess, or a tail gas of a process for making nitric acid withdrawnfrom an absorption column; and the method does not comprise passing thegas through a series of another de-NOx catalytic bed and a subsequentde-N2O catalytic bed, before the passage through said de-NOx catalyticbed.